Global coronal seismology and EIT waves

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Global coronal seismology and EIT waves

• Istvan Ballai
• SP2RC, University of Sheffield, UK

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Coronal seismology

• Local seismology using waves propagating in
  magnetic structures (coronal loops, filaments,
  solar wind, etc)
• Global seismology using waves propagating over
  very large distances in the quiet Sun, e.g. EIT
  waves and the connection
• between global and local waves

Started with Roberts et al (1983), Aschwanden et
al. 1999, Nakariakov et al. 1999 the development
is accelerated and diversified by a large number
of high-resolution observations
Started with Meyer 1968, Uchida 1970, Ballai et
al. 2005, Ballai 2007 is backed by observations
og global waves by, e.g. SOHO, TRACE, STEREO
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EIT waves

• Generated by sudden energy releases (flares,
  CMEs) very well correlated to CMEs, weakly to
  flares
• Observed to propagate over large distances,
  sometimes comparable to the solar radius the
  shape is almost circular (in many cases)
• Large span of velocities (100-400 km/s)
• Able to carry information about the quiet Sun
• Problems with EIT waves
• There is no unified concept about EIT waves
• Most of observations during solar minima
• Not properly observed (see, e.g. Wills-Davey,
  2006)

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Observation of EIT waves

• Although there is a very good correlation not
  every impulsive event is associated with an EIT
  wave

Causes 1. Observational SOHO/EUV

• poor temporal resolution (1 frame/12-15 min)
• not able to record EIT waves for flares/CMEs
  near the limb

TRACE/EUV

• Much better resolution but limited FOV
  (observation of EIT waves is merely a matter of
  luck)
• Wave front too faint to be observed

2. Theoretical

• If the idea of guided trapped waves is OK, waves
  become evanescent very quickly

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EIT wave seen by SOHO/EIT
(courtesy of M. Wills-Davey)
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EIT wave seen by STEREO/EUVI
(Courtesy of G. Attrill)
Propagation speed 28855 km/s
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EIT waves observed by TRACE/EUV
The 13 June 1998 event (Wills-Davey and Thompson
1999, Ballai, Erdélyi and Pintér 2005) 1525 UT
1544 UT
TRACE 195 A (1.5 MK)
Oscillatory motion with periods of about 400
seconds (Ballai, Erdélyi and Pintér 2005)
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Generation of EIT waves

• For simplicity suppose a magnetic-free
  environment, and study the propagation of waves
  at a single spherical interface

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Generation of EIT waves

• The difference in the pressure perturbation in
  the two regions could generate a siphon flow
  which drives much denser material in the outer
  region
• In the exterior (right hand side), the dimming
  propagates away from the source (as observed)

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Sampling the magnetic field (vertical)

• Suppose that EIT waves are FMW propagating
  perpendicular to the ambient magnetic field
  c(cS2vA2)1/2
• The propagation height of EIT waves is important
  since many physical parameters (temperature,
  density, pressure) have height-dependence
• Suppose a simple atmosphere such that (Sturrock
  et al. 1996)

F0 inward heat flux (1.8105 erg/cm2s) x
normalized height coordinate (r/R) T0
temperature at the base of the model (1.3MK) ?
coefficient of thermal conductivity d a constant
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Sampling the magnetic field (vertical) contd...

• With the sound speed and density calculated at
  each height, values of the magnetic field (via
  the Alfvén speed) are obtained to be

x T n cS vA (1) B(1) vA(2) B(2)
1.00 1.30 3.60 1.72 1.81 1.57 3.61 3.13
1.02 1.41 3.30 1.80 1.73 1.44 3.57 2.97
1.04 1.50 3.10 1.85 1.67 1.34 3.54 2.85
1.06 1.58 2.95 1.90 1.61 1.27 3.51 2.76
1.08 1.64 2.83 1.94 1.57 1.21 3.49 2.69
1.10 1.70 2.73 1.97 1.52 1.15 3.47 2.63
T MK n 108 cm-3 cS,vA 107 cm/s B
G (a) c250 km/s (b) c400 km/s
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Flare and magnetic field diagnostics

• EIT waves interact with loops transferring part
  of their energy to loops ? loop oscillations
• Supposing that the entire energy of EIT waves is
  transferred to loops, the minimum energy of EIT
  waves is

• For the event on 13 June 1998, we obtain
  E3.81018 J, for the event on 14 July 1998
  (Nakariakov et al. 1999) we obtain E1019 J.
• Since ?e-1 contains the Alfvén speed, it is
  possible to derive a formula giving the magnetic
  field in the oscillating loop provided the energy
  of the EIT wave can be measured.

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Flare and magnetic field diagnostics

• Time L(Mm) R(Mm) n108(cm-3)
  E(J)
• 980714 168 7.2 5.7 2.2x1017
• 980714 204 7.9 6.2 9.7x1018
• 981123 190 16.8 3 1.3x1019
• 990704 258 7 6.3 3.9x1016
• 991025 166 6.3 7.2 1.6x1018
• 000323 198 8.8 17 5.2x1016
• 000412 78 6.8 6.9 2.5x1016
• 010321 406 9.2 6.2 7.4x1016
• 010322 260 6.2 3.2 1.9x1016
• 010412 226 7 4.4 1.4x1018
• 010415 256 8.5 5.1 1.4x1016
• 010513 182 11.4 4 2.2x1018
• 010515 192 6.9 2.7 1.6x1019
• 010615 146 15.8 3.2 1.1x1017

Lengths, width and densities taken from
Aschwanden et al. (2001) Time given in yymmdd
format E the minimum energy of EIT waves to
generate the observed dislocation of loops No
particular correlation between the energy and
geometrical sizes of loops but a relative good
agreement between energy and 1/n
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Sampling the magnetic field (tangential)Magnetic
map of the quiet Sun
Magnetic tomography of the quiet Sun
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Conclusions

• EIT waves are very good candidates for sampling
  the coronal magnetic field in the quiet Sun
• More observations are needed with higher
  resolution
• Since EIT waves relate flare/CMEs with
  oscillations in coronal loops, they are very
  useful tools to diagnose the magnetic field on a
  larger scale and connect CMEs and loop
  oscillations
• After all, the magnitude of the magnetic field is
  not the most important factor, instead the of
  structure (sub-structure) of the magnetic field
  could be more interesting and important